Currently, portable devices are becoming widespread at a fast pace. In the days ahead, portable devices will further be downsized and thinner, and offer more technical advantages. In this case, there would be many cases where persons hold and manipulate portable devices in their hand over a prolonged period of time. For this reason, it is important to keep surfaces of portable devices at low temperatures.
As an example of a method for preventing a rise in the temperature of the surface of a portable device, a method in which a graphite sheet is placed directly above a heat-generating component inside the portable device can be mentioned. In this method, the heat from the heat-generating component is dissipated through the graphite sheet to even the temperature distribution inside the portable device, thereby suppressing the local temperature rise (PTL 1). However, with advancement in performance of semiconductor chips that are heat-generating components, the surface reaching temperature has been higher than ever before, and the heat countermeasure relying solely on graphite sheets now has limitations.
Hence, a combination of a graphite sheet and a heat-insulating material can be considered. As an example of a heat-insulating material having high heat-insulating performance, silica aerogels can be mentioned.
Silica aerogels have been known as nanoscale porous bodies that have a porosity of 90% or more. Furthermore, silica aerogels are superior to existing heat-insulating materials in terms of curing deterioration and heat resistance, and are known to have an excellent heat conductivity of around 15 mW/mK. However, since a network structure in which silica particles on the scale of several tens of nanometers are connected through point contact is formed in silica aerogels, their mechanical strength is not very high. Therefore, in order to overcome the weakness, attempts to combining silica aerogels with fibers or unwoven fabrics, resins, etc. to improve their strength have been studied.
PTL 2 proposes a method in which a sol of silica aerogel is sprayed onto a fiber material including two components, low-melting-point fibers and high-melting-point fibers, and the fiber material is thermally compressed to produce a heat-insulation material. In this method, the low-melting-point fibers are thermally compressed at a temperature equal to or higher than the melting point to bind the silica aerogels and the fibers, thereby alleviating omissions of aerogels.
Furthermore, with regard to composites of silica aerogels and fiber resins, PTL 3 and PTL 4 have been known.